This invention relates to the separation of p-fluoroaniline from a mixture containing p-fluoroaniline and other aromatic compounds. More specifically there is disclosed a process for the separation of p-fluoroaniline from a mixture containing p-fluoroaniline and aniline comprising contacting the mixture with a mineral acid, in the presence of a solvent, thereafter isolating the resultant salt of p-fluoroaniline from the reaction mixture and then contacting the isolated salt with a base, thereby converting the salt to p-fluoroaniline.
P-fluoroaniline is a well known intermediate in the preparation of dyestuffs, pesticides and pharmaceuticals. U.S. Pat. No. 2,884,458 discloses a process for the manufacture of p-fluoroaniline by catalytic hydrogenation of nitrobenzene in anhydrous hydrogen fluoride. In the catalytic hydrogenation process, large amounts of ordinary aniline accompany the p-fluoroaniline and are difficult to separate. Generally, from 25 to 50 percent or more of the aniline product is aniline and the balance of p-fluoroaniline. Unfortunately, this reference does not provide an efficient means of separation of the p-fluoroaniline.
One method of separating aniline from p-fluoroaniline is disclosed in U.S. Pat. No. 3,639,482 and U.S. Pat. No. 3,580,951. U.S. Pat. No. 3,639,482 discloses a process for preparing fluoroaniline by heating at a temperature of from 100.degree. to 230.degree. C. under a pressure of 15 to 1500 p.s.i.a., a mixture of anhydrous hydrogen fluoride, nitrobenzene and a catalyst mixture consisting essentially of a compound selected from the group of palladium halides, rhodium halides, palladium oxides, and rhodium oxides and mixtures thereof, with an oxide of an element selected from the group of vanadium, molybdenum, tungsten, niobium, chromium, tantalum and mixtures thereof. After the reaction is completed, the product is isolated in a conventional manner. For example, the catalyst is removed by filtration and any excess hydrogen fluoride is evaporated or distilled off. Water is added and the aqueous layer is made alkaline. The liberated anilines are then separated and/or extracted with any suitable water immiscible organic solvent, for example, ether. The extract is dried and distilled to recover the fluoroaniline product. U.S. Pat. No. 3,580,951 describes the separation of the p-fluoroaniline from aniline in essentially the same manner as disclosed in U.S. Pat. No. 3,639,482.
U.S. Pat. No. 3,558,707 discloses a process for preparing fluoroanilines. The fluoroanilines are prepared by the deoxygenation and hydrofluorination of the corresponding nitrobenzenes which are reacted in anhydrous hydrogen fluoride at 0.degree. to 230.degree. C. under pressures of from 15 to 1500 p.s.i.a. in the presence of certain deoxygenating agents containing phosphorous or sulfur. After the reaction is completed, the fluoroaniline product is isolated in a conventional manner. For example, excess hydrogen fluoride is evaporated or distilled off. Water is added and the oxidation product together with unconverted deoxygenating agent, both of which are usually insoluble in water, are filtered off or otherwise separated. The aqueous layer is made alkaline and the liberated anilines are separated and/or extracted with any suitable water immiscible organic solvent, for example, ether. The extract is dried and distilled to recover the fluoroaniline product.
All of the prior art methods described above rely on fractional distillation to separate anilines from the corresponding fluoroanilines. Unfortunately, at atmospheric pressures aniline and p-fluoroaniline boil within about 3.degree. C. of each other, which makes fractional distillation very difficult. Accordingly, there exists a demand for an efficient means of separating p-fluoroaniline from mixtures containing p-fluoroaniline and aniline.